System for electromagnetic field conversion

ABSTRACT

A system is provided for conversion of a first electromagnetic field into a desired second electromagnetic field, for example for coupling modes between waveguides or into microstructured waveguides. The system includes a complex spatial electromagnetic field converter that is positioned for reception of at least a part of the first electromagnetic field and that is adapted for conversion of the received field into the desired electromagnetic field, and wherein at least one of the first and second fields matches a mode of a microstructured waveguide. An advantage of the system is that the full effect of an incident light beam may be utilized for exciting a desired complicated mode of a specific waveguide. Another advantage is that the power of the incident beam may be coupled into one specific mode and not others, whereby a high mode suppression ratio may be achieved.

This application claims the benefit under 35 U.S.C. 119(e) of U.S.Provisional Application No. 60/329,497, filed on Oct. 17, 2001, and U.S.Provisional Application No. 60/339,104, filed on Dec. 13, 2001.

FIELD OF THE INVENTION

The present invention relates to a system for conversion of a firstelectromagnetic field to a desired second electromagnetic field, forexample for coupling of an electromagnetic field into an electromagneticwaveguide.

BACKGROUND OF THE INVENTION

In order to couple light emitted by a light emitting diode or asemiconductor laser into an optical fiber, such as a single mode fiber,it is well known to use a butt coupling or a lens coupling. The buttcoupling is a direct coupling Wherein the fiber is brought close to thelight source. The butt coupling provides only about 10% efficiency for alaser, as it makes no attempt to match the mode sizes of the laser andthe fiber. For example, the laser mode size may be about 1 μm and themode size of a single mode fiber may be in the range 6-9 μm. Thecoupling efficiency may be Improved by tapering the fiber end andforming a lens at the fiber tip.

In a lens coupling design, the coupling efficiency can exceed 70% for aconfocal design in which a sphere is used to collimate the laser lightand focus it onto the fiber core. The alignment of the fiber core isless critical for the confocal design because the spot size is matchedto the fiber's mode size.

These coupling approaches are well suited for excitation of first ordermodes in optical fibers since the phase of a first order mode does notvary over the fiber cross-section, rather it fits with anelectromagnetic field with a symmetric phase and amplitude wave front,such as the electromagnetic field in a light beam emitted by a laser.The phase distribution of a second-order mode, however, is usuallysymmetric in magnitude but changes sign about a symmetry axis of thefiber, and for higher order modes the phases change sign several timesacross the cross-section of the fiber. In order to excite higher ordermodes, the incident electromagnetic beam is typically focused on a partof the cross-section of the fiber, namely an area within which the phasedoes not change sign. This limits the obtainable coupling efficiency toa value that is roughly equal to the ratio between the illuminated areaand the total cross-sectional area of the fiber core. This may be seenby the overlap integral (or inner product) between the two mode plots.

In “Selective launching of higher-order modes into an optical fiber withan optical phase shifter”, by W. Q. Thornburg, B. J. Corrado, and X. D.Zhu, Optics Letters, vol. 19, No. 7, Apr. 1, 1994, a coupling approachis disclosed for exciting a second order mode in a weakly guided,cylindrically symmetric step-index fiber by phase shifting one bisectionof the beam so that polarization and phase front of the incident beammatches the desired mode.

As for example disclosed in “Crystal fiber technology”, Jes Broeng, StigE. Barkou, Anders Bjarklev, Thomas Søndergaard, and Erik Knudsen,DOPS-NYT 2-2000, and “Waveguidance by the photonic band gap effect inoptical fibres”, Jes Broeng, Stig E. Barkou, Anders Bjarklev, ThomasSøndergaard, and Pablo M Barbeito, recently, a new approach of makingoptical fibers has been invented by Professor Philip Russell and histeam at the Department of Physics at the University of Bath. In anoptical fiber produced according to the new approach, bundles ofmicroscopic dielectric pipes extend along a longitudinal axis of thefiber. Thus, a cross-section of the fiber exhibits holes arranged in anarray like atoms in a crystal, hence the name crystal fibers also knownas microstructured or holey fibers. The dielectric may be silica, dopedsilica, polymers, etc.

In index-guided crystal fibers, one or more holes are missing at thecenter of the array. Without the holes, the glass at the center has ahigher density than its surroundings, and light entering the center,i.e. the core, is therefore confined much as it would be in aconventional fiber. The advantage is that the effect is achieved withoutnecessarily having to use two different kinds of glass. An added benefitis that the light can be squeezed into a much narrower core than is thecase in conventional fibers, or large mode area single mode fibers canbe made. There is a great taylorability of mode size and general modeproperties in a photonic crystal fiber (PCF).

In photonic crystal fibers operating by photonic band gap effect (PBGfibers), the holes are arranged in a photonic crystal with band gapswherein no modes can propagate through tho fiber. By locally breakingthe periodicity of the photonic crystal, a spatial region with opticalproperties different from the surrounding bulk photonic crystal can becreated. If such a defect region supports modes with frequencies fallinginside the forbidden gap of the surrounding full-periodicmicrostructure, these modes will be strongly confined to the defect. Itis important to note that it is not a requirement that the defect regionhas a higher index than its surroundings. If the surrounding materialexhibits photonic band gap effects, even a low-index defect region isable to confine light and thereby act as a highly unusual waveguide. Thedefect may be an air filled tube that may provide—in theory—no lossguidance over long distances.

It has been shown that photonic crystal fibers may support single modeoperation in a larger wavelength range than conventional fibers. e.g.from UV light to mid-infrared wavelengths, i.e. the entire wavelengthrange where silica can be used, and that photonic crystal fibers can bedesigned with a very flat near-zero dispersion over a very broadwavelength range. Further, photonic crystal fibers may be produced withvery large positive dispersion for single-mode operation, This may beutilized for dispersion management in fiber systems with negativedispersion or vice versa.

Photonic crystals are structures having a periodic variation indielectric constant. The dielectric may be silica, doped silica,polymers, etc. By fabricating photonic crystals having specificperiodicities, the properties of the photonic band gap can be designedto specific applications. For example, the central wavelength of aphotonic band gap is approximately equal to (in order of magnitude) theperiodicity of the photonic crystal and the width of the photonic bandgap is proportional to the differences in dielectric constant within thephotonic crystals, For a general reference, see: J. D. Joannopoulos etal., Photontic Crystals, Princeton University Press, Princeton, 1995. Byinclusion of defects with respect to their periodicity in photoniccrystals, a localized electromagnetic mode having a frequency within aphotonic band gap may be supported. For example, in a three-dimensionalphotonic crystal formed by dielectric spheres at the sites of a diamondlattice, the absence of a sphere produces a defect. In the immediatevicinity of the absent sphere, the photonic crystal is no longerperiodic, and a localized electromagnetic mode having a frequency withinthe photonic band gap can exist. This defect mode cannot propagate awayfrom the absent void, it is localized in the vicinity of the defect.Thus, the introduction of a defect into the photonic crystal creates aresonant cavity, i.e. a region of the crystal that confineselectromagnetic radiation having a specific frequency within the region.A series of defects can be combined to form a waveguide within thephotonic crystal. Such waveguides in photonic crystals can include sharpturns, such as 90° bends substantially without loss. For example, U.S.Pat. No. 5,526,449 discloses waveguides based on photonic crystals forincorporation into opto-electronic integrated circuits.

The crystal fibers previously mentioned are examples of two-dimensionalphotonic crystals with electromagnetic mode supporting defects. A largevariety of design options is available to the designer of crystalfibers. By careful selection of preform tube geometry, tube density,tube positions, and utilization of tubes of different types in the samefiber, the designer can provide waveguides with desired characteristics,such as transmission loss, dispersion, non-linearity, mode structure,micro- and macro-bend loss, etc. Examples of various designs aredisclosed in WO 99/64903, WO 99/64904, and WO 00/60390.

Examples of one-dimensional photonic crystals are given in U.S. Pat. No.6,130,780 disclosing an omni-directional reflector with a surface and arefractive index variation along the direction perpendicular to thesurface so that a range of frequencies exists defining a photonic bandgap for electromagnetic energy incident along the perpendiculardirection to the surface The structure further fulfils a criterion bywhich no propagating states may couple to an incident wave and thus thedielectric structure acts as a perfect reflector in a given frequencyrange for all incident angles and polarizations.

In WO 00/65386, an all-dielectric coaxial waveguide is disclosed that isdesignated a coaxial omniguide and that is based on the omni-directionaldielectric reflector disclosed in U.S. Pat. No. 6,130,780. The radialconfinement of the light in the coaxial omniguide is a consequence ofomni-directional reflection and not total internal reflection. Thismeans that the coaxial omniguide can be used to transmit light aroundmuch sharper corners than the optical fiber. Also, the radial decay ofthe electromagnetic field in the coaxial omniguide is much greater thanin the case of the optical fiber so that the outer diameter of thecoaxial omniguide can be much smaller than the diameter of the claddinglayer of the optical fiber without leading to cross-talk.

SUMMARY OF THE INVENTION

In the following a microstructured waveguide designates a one-, two- orthree-dimensional photonic crystal with defects for propagation of anelectromagnetic field and optionally with interstitial voids, such asindex-guided crystal fibers, photonic band gap crystal fibers, coaxialomniguides, polymer optical fibers, polymer crystal fibers, holeassisted light guide fibers (e.g. as disclosed in “Modeling and designoptimization of hole-assisted lightguide fiber by full-vector finiteelement method”, by T. Hasegawa et. al. Proc. 27^(th) Eur. Conf. On OptComm. ECOC'01 —Amsterdam), hollow optical fiber (e.g. as disclosed in “Anew mode converter based on hollow optical fiber for gigabit LANcommunication”, S. Choi et. al., Proc. 27^(th) Eur. Conf On Opt. Comm.,ECOC'01 —Amsterdam), waveguides in integrated optical circuits, such asphotonic crystal based planar waveguides, a slab waveguide structure,etc, a surface plasmon polariton based waveguide, resonators, coupledcavity waveguides, coupling resonator optical waveguides, photonic Wirewaveguides (ie. very tightly confined waveguides), couplers, powerspiltters, combiners, e.g. 3 dB couplers, etc. A microstructuredwaveguide may transmit an electromagnetic field passively, or may formpart of an active component, e.g. a rare earth doped fiber amplifier,such as an Er doped fiber amplifier, an Yb doped fiber amplifier, etc, aRaman amplifier, a Brilouin amplifier, etc.

When a desired mode propagating through a microstructured waveguide isof a high order, the phase of the propagating electromagnetic field orwave changes sign at least once across the cross-section of thewaveguide. As previously described, such a mode is typically excited byfocusing an incident light beam on an area of the waveguide end withinwhich the phase does not change its sign.

Thus, there is a need for a coupling approach that can excite a desiredmode in a microstructured waveguide with a high coupling efficiency.There is also a need for a system that excites a desired mode withoutexciting other modes, i.e. to Suppress other modes than the desired onewhile still keeping an efficient coupling to the desired mode.

Likewise, there is a need for an approach of converting a high ordermode emitted from a microstructured waveguide to a mode that matches amode in a conventional waveguide, such as a single mode step indexfiber, a graded index fiber, such as a parabolic index fiber, e.g. amultimode parabolic index fiber.

According to the present invention this and other objects are fulfilledby utilization of a complex spatial electromagnetic field converter forconversion of a given first electromagnetic field into a desired secondelectromagnetic field.

At least one of the fields may match a mode of a microstructuredwaveguide.

For example, the first electromagnetic field may be emitted from anoutput end of the microstructured waveguide.

For example, a light beam emitted by a semiconductor laser may beconverted into a second electromagnetic field that matches a mode of amicrostructured waveguide.

In the present disclosure, an electromagnetic field is said to match amode of a microstructured waveguide when the electromagnetic field inquestion can excite the mode of the waveguide with a coupling efficiencythat exceeds the ratio between an area of the cross-section of thewaveguide within which the phase of the mode in question does not changeits sign and the entire cross-section of propagating region of thewaveguide.

In a preferred embodiment of the invention the matching field issubstantially equal to the field of the mode in question.

It is an important advantage of the present invention that the fulleffect of an incident light beam may be utilized for exciting a desiredmode of a specific waveguide since the illumination of the waveguide endis no longer required to be confined to a part of the waveguide endwithin which the phase of the mode in question does not change its sign.

It is another Important advantage that the power of the incident beammay be coupled into one specific mode and not others whereby a high modesuppression ratio may be achieved.

The electromagnetic radiation may be of any frequency range of theelectromagnetic spectrum, i.e. the gamma frequency range, theultraviolet range, the visible range, the infrared range, thetelecommunication band or bands, the far infrared range, the X-rayrange, the microwave range, the HF (high frequency) range, etc.

Preferably, the electromagnetic radiation is generated by a coherentsource of electromagnetic radiation, such as a laser, a semi-conductorlaser, a strained multi-quantum well laser, a vertical cavity surfaceemitting laser (VCSEL), a maser, a phase-locked laser diode array, alight emitting diode, a pulsed laser, such as a sub-picosecond laser,etc. However a high pressure arc lamp, such as a Hg lamp, a Xe lamp etc,may also be used and even an incandescent lamp may be used as a sourceof electromagnetic radiation.

The complex spatial electromagnetic field converter may modulate animpinging field by reflection, refraction, or diffraction or anycombination hereof, Further, the complex spatial electromagnetic fieldconverter may modulate phase, amplitude, polarization, or mode fielddiameter, or any combination hereof.

The complex spatial electromagnetic field converter may comprise arefractive element, such as a refractive element with a surfacestructure providing the desired phase modulation, e.g. a surface etchedstructure, or a lenslet array, a ball lens, a semi-ball lens, anaspheric lens or a lens that is not circular symmetric, an-amorphicoptics, mirrors that are deformed to provide the desired phasemodulation, a refractive phase plate, a GRIN (graded index) material,beamsplitter, etc.

The complex spatial electromagnetic field converter may comprise aspatial light modulator.

The spatial light modulator may comprise resolution elements (x, y),each resolution element (x, y) modulating the phase and/or the amplitudeof electromagnetic radiation incident upon it with a predeterminedcomplex value a(x, y)e^(|φ(x, y)), i.e. the amplitude of theelectromagnetic field incident upon the resolution element (x, y) ismultiplied by a(x, y) and φ(x, y) is added to the corresponding phase.The amplitude modulation a(x, y) may be set to unity to obtain a phasemodulation, and φ(x, y) may be sot to zero to obtain an amplitudemodulation, Further, the spatial light modulator may modulate thepolarization of the incoming electromagnetic field by selectivelymodulating vector components of the field individually by eachresolution element (x, y).

Each resolution element may be addressed either optically orelectrically. The electrical addressing technique resembles theaddressing technique of solid-state memories in that each resolutionelement can be addressed through electronic circuitry to receive acontrol signal corresponding to the phase and/or amplitude change to begenerated by the addressed resolution element. The optical addressingtechnique addresses each resolution element by pointing a light beam onit, the intensity of the light beam corresponding to the modulationchange to be generated by the resolution element illuminated by thelight beam.

The Spatial light modulator (SLM) may be a fixed phase mask, a liquidcrystal device based on liquid crystal display technology, a MEMS (microelectro-mechanical system), a MOEMS (micro opto-electro-mechanicalsystem), such as a dynamic mirror device, a digital micro-mirror array,a deformable mirror device, etc, a membrane spatial light modulator, alaser diode array (integrated light source and phase modulator), smartpixel arrays, etc.

Selko-Epson produces a transmitting liquid crystal SLM (LC-SLM) having ahigh resolution matrix of transparent liquid crystal elements whereinthe relative permittivity of each element can be electrically modulatedin order to vary the refractive index and thereby the optical pathlength of the element. Meadowlark produces a parallel-aligned liquidcrystal (PAL-SLM) with a high fill factor, but this device has a verylow resolution in that it contains only 137 phase-modulating elements.

Hamamatsu Photonics produces a dynamically controllable PAL-SLM with VGAor XGA resolution.

Texas Instruments produces a Digital Mirror Device (DMD) having an arrayof mirrors each of which can be tilted between two positions.

The complex spatial electromagnetic field converter may comprise adiffractive optical element (DOE), e.g a holographic optical element(HOE), A DOE operates on the principle of diffraction, Traditionaloptical elements use their shape to bend light. DOEs comprisediffractive gratings or fringe patterns that, in response to an incidentwave, generate a plurality of electromagnetic waves which recombine toform the desired waves A grating or a fringe pattern may be a lattice ofpoint or line scatterers and/or a lattice of similar refractive indexmodulations.

DOEs can function as gratings, lenses, aspherics or any other type ofoptical element. Large optical apertures, light weight and lower costare the main features of DOEs. They can offer unique optical propertiesthat are not possible with conventional optical elements.

Several different optical elements can share the same substrate withoutinterfering with one another, Thus, a single DOE can be used as a lens,beam splitter and spectral filter simultaneously.

Diffractive elements are very light, as they are formed in thin films ofa few μm thickness only. A diffractive element can be fabricated on anyarbitrary shape of the substrate. They can be made to operate over anarrow wavelength band.

The fabrication and replication of DOEs are relatively easy and cheapbecause no precision shaping of a surface is required.

By using real-time recyclable recording media, any desired systemfunction can be recorded and erased repeatedly.

In traditional holography an interference between an object beam and areference beam is created and recorded on a photographic emulsion.

More than one independent interference pattern can be stored in the samerecording media without any cross-talk.

A hologram may be of an absorption type Which produces a change in theamplitude of the reconstruction beam. The phase type hologram producesphase changes in the reconstruction beam due to a variation in therefractive index or thickness of the medium. Phase holograms have theadvantage over amplitude holograms of no energy dissipation within thehologram medium and higher diffraction efficiency. Holograms recorded inphotographic emulsions change both the amplitude and the phase of theilluminating wave. The shape of the recorded fringe planes depend on therelative phase of the interfering beams.

A volume (thick) hologram may be regarded as a superposition ofthree-dimensional gratings recorded in the depth of the emulsion eachsatisfying the Bragg condition. The grating planes in a volume hologramproduce maximum change in refractive index and/or absorption index. Aconsequence of Bragg condition is that the volume hologram reconstructsthe virtual image at the original position of the object if thereconstruction beam exactly coincides with the reference beam. However,the conjugate image and higher order diffractions are absent.

Holographic optical elements may comprise interferometrically generatedholograms. computer-generated holograms including kinoforms, E-beamwritten holograms, edge-illuminated holograms, waveguide coupledholograms, deep surface relief holograms, micro-machined holograms, andFresnel zone plates.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the general principle of present invention,

FIG. 2 shows a 4 f optical system for electromagnetic field conversionin accordance with the present invention,

FIG. 3 shows another 4 f optical system for electromagnetic fieldconversion in accordance with the present invention,

FIG. 4 schematically shows a combination of the system illustrated inFIG. 3 with the system illustrated in FIG. 11,

FIG. 5 schematically shows the microstructure of an exemplary photoniccrystal fiber,

FIG. 6 schematically shows the cross-sectional phase distribution of apropagating mode of the photonic crystal fiber of FIG. 5,

FIGS. 7, 8 illustrate utilization of an analog hologram as a complexspatial electronmagnetic field converter,

FIGS. 9, 10 illustrate utilization of a volume hologram as a complexspatial electromagnetic field converter,

FIG. 11 shows another 4 f optical system wherein the complex spatialelectromagnetic field converter 4 is positioned in the Fourier plane ofthe first Fourier transforming lens 5,

FIG. 12 shows an optical system for electromagnetic field conversionwith one lens,

FIG. 13 shows a system according to the present invention comprising aplurality of the complex spatial electromagnetic field converters 4, 4′,

FIG. 14 shows a system according to the present invention integratedinto a waveguide module, and

FIG. 15 illustrates a complex spatial electromagnetic field converterthat is integrated with an end facet of a waveguide.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 illustrates the general principle of the present inventionwherein the first electromagnetic field is generated by superposition ofelectromagnetic fields emitted from a first set of waveguides 1, 2, . .. , m. The system 1 comprising the complex spatial electromagnetic fieldconverter converts the first electromagnetic field into the desiredsecond electromagnetic field that is the superposition of desiredpropagating modes of a second set of waveguides 1, 2, . . . , n. Thesystem operates to perform both mode conversion and switching.

FIG. 2 shows a 4 f optical system 1 for conversion of a firstelectromagnetic field 6, namely a light beam emitted by a laser 2 andcollimated by the collimator 3, into a desired second electromagneticfield 8 for propagation through the microstructured waveguide 10. Acomplex spatial electromagnetic field converter 4, such as a spatiallight modulator (SLM), is positioned for reception of the firstelectromagnetic field 6 that is transmitted through tile complex spatialelectromagnetic field converter 4 and a Fourier transforming lens 5having a focal length f₁. The complex spatial electromagnetic fieldconverter 4 is positioned in the front focal plane of the lens 5.Another Fourier transforming lens 7 with a focal length f₂ is positionedso that its front focal plane coincides with the back focal plane oflens 5 as is well known in 4 f optical systems. The magnification of thesystem is f₂/f₁. The converted electromagnetic field 8 is generated inthe back focal plane 9 of the lens 7 and input to the microstructuredWaveguide 10. It is seen that the surface of the complex spatialelectromagnetic field converter 4 is imaged onto the end surface of thewaveguide 10 by the Fourier transforming lenses 5, 7, e.g. (x, y) isimaged onto (x′, y′) at the end of the waveguide 10. As previouslydescribed, each resolution element (x, y) of a spatial light modulatormodulates the phase and the amplitude of electromagnetic radiationincident upon it with a predetermined complex value a(x, y)e^(|φ(x, y)).Further the spatial light modulator may modulate the polarization of theincoming electromagnetic field by selectively modulating vectorcomponents of the field individually by each resolution element (x, y).Thus, the values of a(x, y) and φ(x, y) for each vector component aredetermined from the amplitude and phase values at correspondingpositions (x′, y′) at the waveguide end of the desired waveguide modewhereby the collimated electromagnetic field 6 is converted into thedesired electromagnetic field 8 that matches a desired mode of themicrostructured waveguide 10.

The system 1 may be simplified by positioning of the complex spatialelectromagnetic field converter 4 in the Fourier plane of lens 5, i.e.the front focal plane of lens 7, and removal of lens 5. This requiresthat the complex spatial electromagnetic field converter 4 converts theincoming electromagnetic field 6 into the Fourier transformed field ofthe desired mode of the waveguide 10 since the lens 7 generates aFourier transformation of the field at the output surface of the complexspatial electromagnetic field converter 4. In this case the resolution,i.e. number of resolution elements, of the complex spatialelectromagnetic field converter 4 must be much higher than for the 4 fsystem of FIG. 1.

The lenses 5, 7 may be compound lenses, doublets, achromats, f-thetalenses. microscope lenses, microscope objectives, graded-Index lenses,aspherical lenses and/or non-circularly symmetrical lenses, etc.Further, the lenses 5, 7 may be ball lenses offering a system of a smallsize.

The complex spatial electromagnetic field converter 4 may be a spatiallight modulator (SLM), such as a phase-only spatial light modulator(POSLM) wherein the amplitude of the field is not modulated, a complexspatial light modulator modulating amplitude and phase, or apolarization modulator also modifying the field vector components of theelectromagnetic field.

The microstructured Waveguide may be an index-guided crystal fiber,photonic band gap crystal fiber, coaxial omniguide, polymer opticalfiber, polymer crystal fiber, hole assisted light guide fiber, hollowoptical fiber, waveguides in integrated optical circuits, such asphotonic crystal based planar waveguides, a slab waveguide structure,etc, a surface plasmon polariton based waveguide, resonators, coupledcavity waveguides, coupling resonator optical waveguides, photonic wirewaveguides (i.e. very tightly confined waveguides), couplers,powersplitters, combiners, e.g. 3 dB couplers, etc, A microstructuredwaveguide may transmit an electromagnetic field passively or may formpart of an active component, e.g. a rare earth doped fiber amplifier,such as an Er doped fiber amplifier, an Yb doped fiber amplifier, etc,Raman amplifier, Brillouin amplifier, etc.

It is obvious that other systems according to the present invention maybe designed with optical components in Fresnel planes.

FIG. 3 shows a 4 f optical system similar to the system shown in FIG. 2,however in FIG. 3 the electromagnetic field 6 to be converted is emittedby a microstructured waveguide 10. The complex spatial electromagneticfield converter 4 is adapted to convert the mode of the microstructuredwaveguide 10 into the mode of the single mode step index fiber 12, Ofcourse the single mode step index fiber 12 may be substituted with anyof the fibers mentioned above.

FIG. 4 combines the system illustrated in FIG. 3 with the systemillustrated in FIG. 11 whereby system requirements of each of thecomplex spatial electromagnetic field converters may be lowered comparedto the previously illustrated systems. For example, POSLMs may be usedFor provision of both amplitude and phase modulation.

FIG. 5 schematically shows the microstructure of an exemplary photonicband gap crystal fiber, and FIG. 6 schematically shows thecross-sectional phase distribution of a propagating mode of the photonicband gap crystal fiber, it is seen that in this case the phase changessign six times as a function of the angular position in a cross-sectionof the fiber. In a preferred embodiment of the invention, the complexspatial electromagnetic field converter 4 is dynamically adjustable. Forexample, the resolution elements (x, y) of a spatial light modulator maybe addressed so that the modulating values of a(x, y) and φ(x, y) can beadjusted. In this way the modulation pattern a(x, y)e^(|φ(x, y)) of thespatial light modulator may be rotated until its phase pattern coincideswith the phase pattern of the mode of the waveguide 10 either in thecase wherein the first electromagnetic field 6 is emitted by thewaveguide 10 or wherein the converted second field is coupled into thewaveguide 10. Also the modulation pattern may be adjusted to selectivelymatch different desired modes of the waveguide 10, or a desired mode mayselectively be turned on or off with a powerful suppression of possibleother modes if desired.

It should be noted that the illustrated propagating mode of FIG. 4 is anexample. Fibers may be provided with propagating modes with an arbitrarynumber of phase changes radially and tangentially across a cross-sectionof the fiber.

FIGS. 7 and 8 illustrate utilization of an analog hologram as a complexspatial electromagnetic field converter. In FIG. 7, light 14 of adesired mode of a microstructured waveguide 10 is emitted from the endof the waveguide 10 and is collimated by the lens 20 and impinges on thesurface of the hologram 22 for interference with a collimated referencebeam 24. The reference beam may be emitted by a semiconductor laser, byanother microstructured waveguide, a conventional optical fiber, etc. InFIG. 8, the desired mode is excited in the waveguide 10 by emitting aconjugated reference beam 26 towards the hologram 22 whereby thecollimated electromagnetic field 16 of the desired mode is regeneratedfor coupling into the waveguide 10. Obviously, the fringe pattern of thehologram 22 may be computer generated thus, eliminating the need for theoptical recording set-up illustrated in FIG. 7.

FIGS. 9 and 10 illustrate utilization of a volume hologram 22 as acomplex spatial electromagnetic field converter. In FIG. 9, light 14 ofa desired mode of a microstructured waveguide 10 is emitted from the endof the waveguide 10 and is collimated by the lens 20 and impinges on thehologram 22 for interference with a collimated reference beam 241.Different desired modes of the waveguide 10 may be recorded on thevolume hologram 22 with different respective reference beams 24 ₁, 24 ₂,. . . , 24 _(n). Again, the reference beam may be emitted by asemiconductor laser, by another microstructured waveguide, aconventional optical fiber, etc. In FIG. 10, one of the desired modes isselectively excited in the waveguide 10 by emitting the correspondingconjugated reference beam 26 _(i) towards the hologram 22 whereby thecollimated electromagnetic field 16 of the desired mode is regeneratedfor coupling into the waveguide 10.

In a diffractive optical element, electromagnetic field convertingfringe patterns may be combined with other functional fringe patterns,such as beam splitting fringe patterns. Thus, the incoming field 6 maybe generated by several waveguides, and likewise the convertedelectromagnetic field may be directed towards a plurality of waveguidesand, in combination with such a diffractive optical element, waveguidecouplers, switches, etc. may be provided. A dynamic optical element thatis recorded in a dynamically rewriteable medium may provide dynamicswitching between waveguides.

FIG. 11 shows another 4 f optical system wherein the electromagneticfield 6 to be converted is emitted by a microstructured waveguide 10,and the complex spatial electromagnetic field converter 4 is positionedin the Fourier plane of the first Fourier transforming lens 5 whichcoincides with the front focal plane of second lens 7. The complexspatial electromagnetic field converter 4 multiples the collimatedelectromagnetic field with a filter function a(x, y)e^(|φ(x, y)) thathas been predetermined so that the Fourier transformed of the Fouriertransformed incoming field 6 times the filter function matches thedesired mode of the coaxial omniguide 30.

In FIG. 11, the coaxial omniguide may be replaced by a detector, and thefilter function of the complex spatial electromagnetic field converter 4may be a correlator function providing a peak output when the incomingcollimated field 6 matches the correlator function. This may be utilizedin waveguide sensing systems wherein the propagating mode of thewaveguide 10 is changed in response to a specific influence. The changemay be detected utilizing an appropriate correlator function, e.g. Inrelation to detection of strain, rotation, tilt, off-set, temperature,etc. In hollow core waveguides, such as air core photonic crystalfibers, hole assisted light guide fibers, single hole core doped fibers,etc, this may be utilized for detection of presence of specificsubstances, pressure detection, etc.

In FIG. 12, the electromagnetic field 6 to be converted is emitted by amicrostructured waveguide 10. The complex spatial electromagnetic fieldconverter 4 is arranged perpendicular to the longitudinal axis of thewaveguide 10. The electromagnetic field 6 emitted by the waveguidebroadens into an expanded region as it emerges from the waveguide. Whenthe field 6 passes through the complex spatial electromagnetic fieldconverter 4 the amplitude and/or phase is changed. A focusing lens 7focuses the field into the coaxial omniguide 30.

FIG. 13 shows a system according to the present invention comprising aplurality of the complex spatial electromagnetic field converters 4, 4′.The electromagnetic field emitted by a microstructured waveguide 10 iscollimated by lens 5 and then it passes through two complex spatialelectromagnetic field converters 4, 4′ and is finally focused by lens 7into the coaxial omniguide 30.

FIG. 14 schematically shows a system according to the present inventionintegrated into a waveguide module. The integration is providedutilizing GRIN lenses 34 and a micro-hologram 35. All of the previouslysuggested systems may be integrated in one waveguide coupling module,such as a flip-flop module for a wafer with integrated waveguide(s), or,a fiber coupling module that may be fused to e.g. optical fibers, etc.

FIG. 15 illustrates a complex spatial electromagnetic field converter 38that is integrated with the end facet 36 of the microstructuredwaveguide 10 or alternatively, with the end facet of the single modefiber 32. In the figure, the converter 38 is shown separated from theend facet 36 for clarity only. In an operating system, the converter 38is positioned at the end facet 36, and the microstructured fiber 10 andthe single mode fiber 32 are fused together, e.g. by gluing. The phasevariations 37 of the mode propagating through the microstructured fiber10 is illustrated at the end facet 36 in the same way as in FIG. 6. Thecomplex spatial electromagnetic field converter 38 is adapted to convertthe mode of the microstructured waveguide 10 into the mode of the singlemode step index fiber 32. Thus, light may travel from the single modefiber 32 towards the microstructured waveguide 10, or, light may travelfrom the microstructured waveguide 10 towards the single mode fiber 32.In the illustrated example, the phase shift of the converter 38 is equalto π, i.e. the difference in travelling distance of an electromagneticfield propagating through an area marked with π and an electromagneticfield propagating through a surrounding area is half a wavelength.However, a specific substance, e.g. the substance of the microstructuredfiber 10 may be deposited onto the and facet 36 of the fiber 10, or, thefiber 32, with a height profile that provides the phase shifts neededfor the desired electromagnetic field conversion. In the illustratedexample, a stepped height profile is indicated but it is obvious that aheight profile of any desired shape may be provided. Since the height issmall, i.e. on the order of half a wavelength, the height profile doesnot mechanically influence the fusing of the two fibers 10, 32.Alternatively, a fiber may be cleaved to provide the desired heightprofile at the end facet of the fiber.

The desired conversion may also be provided by provision of a materialwith a desired refractive index profile at the end facet of the fiber inquestion without changing the surface of the end facet, i.e. without aheight or a depth profile, for example by doping of the material at theend facet.

The desired phase shifting may also be provided by removal, e.g.etching, of material from the end facet 37 of the microstructured fiber10, or, from the end facet of the fiber 32, with a depth profileproviding the desired phase shift. Further, the added or removedmaterial may have a desired refractive index profile and may bebirefringent so that, in combination with a desired height or depthprofile, any desired phase, amplitude, and polarization conversion maybe provided.

1. A system for conversion of modes of electromagnetic fieldspropagating through waveguides, comprising: a microstructured waveguidefor propagation of an electromagnetic field with a phase that changessign across a cross-section of the microstructured waveguide; and acomplex spatial electromagnetic field converter, that is positionedexternal to the microstructured waveguide, for reception of at least apart of a first electromagnetic field for modulation of the receivedfield into a second electromagnetic field, and wherein at least one ofthe first and second electromagnetic fields matches a mode of themicrostructured waveguide having a phase that changes sign across thecross-section of the microstructured waveguide.
 2. A system according toclaim 1, wherein the microstructured waveguide is a microstructuredoptical fiber.
 3. A system according to claim 2, wherein the opticalfiber is a photonic band gap fiber.
 4. A system according to claim 1,wherein the microstructured waveguide is a part of an integrated opticaldevice.
 5. A system according to claim 1, wherein the complex spatialelectromagnetic field converter comprises resolution elements (x, y),each resolution element (x, y) modulating phase and amplitude ofelectromagnetic radiation incident thereon with a predetermined complexvalue a(x, y)e^(iφ(x, y)).
 6. A system according to claim 5, wherein thecomplex spatial electromagnetic field converter further modulatespolarization of the electromagnetic radiation incident thereon.
 7. Asystem according to claim 5, wherein each resolution element isaddressed electrically.
 8. A system according to claim 5, wherein eachresolution element is addressed optically.
 9. A system according toclaim 5, wherein the complex spatial electromagnetic field converter isa spatial light modulator.
 10. A system according to claim 1, whereinthe complex spatial electromagnetic field converter comprises adiffractive optical element.
 11. A system according to claim 9, whereinthe complex spatial electromagnetic field converter comprises aholographic optical element.
 12. A system according to claim 11, whereinthe holographic optical element is a Fresnel holographic opticalelement.
 13. A system according to claim 1, wherein the firstelectromagnetic field is generated by at least two light sources.
 14. Asystem according to claim 1, wherein the second electromagnetic fieldmatches desired propagating modes of at least two waveguides.
 15. Asystem according to claim 13, wherein the second electromagnetic fieldmatches desired propagating modes of at least two waveguides.
 16. Asystem according to claim 13, further providing switching.
 17. A systemaccording to claim 1, wherein the complex spatial electromagnetic fieldconverter is dynamically adjustable.
 18. A system according to claim 1,wherein the second electromagnetic field is rotatable for matching ofthe second electromagnetic field with the mode of the microstructuredwaveguide.
 19. A system according to claim 1, comprising GRIN lenses anda micro-hologram for integration of the system into a waveguide module.20. A system according to claim 19, wherein the waveguide module is afiber coupling module fusible to optical fibers.
 21. A system accordingto claim 10, wherein the complex spatial electromagnetic field convertercomprises a Fresnel diffractive optical element.
 22. A system accordingto claim 10, wherein the complex spatial electromagnetic field convertercomprises a Fourier diffractive optical element.
 23. A system accordingto claim 1, wherein the complex spatial electromagnetic field converteris integrated with an end facet of a waveguide.
 24. A system accordingto claim 23, wherein a substance is situated at the end facet of thewaveguide with a refractive Index profile that provides phase andamplitude shifts needed for the electromagnetic field modulation.
 25. Asystem according to claim 24, wherein the substance is deposited ontothe end facet of the waveguide with a height profile that provides thephase and amplitude shifts needed for the electromagnetic fieldmodulation.
 26. A system according to claim 23, wherein a substance isdeposited onto the end facet of the waveguide with a height profile thatprovides phase and amplitude shifts needed for the electromagnetic fieldmodulation.
 27. A system according to claim 26, wherein the substance isbirefringent for provision of a desired polarization shift.
 28. A systemaccording to claim 23, wherein material is removed from the end facet ofthe waveguide with a depth profile providing phase, amplitude, andpolarization shift for the electromagnetic field modulation.